Rank 1

Rank 1 spells are bought from vendors in Maiden's Eye. In Terror of Luclin long duration buffs are no longer ranked, instead are simply sold along with the Rank 1s.

Rank 2

Throughout the expansion trash (rarely), rares (commonly), and chests (always?) drop Minor-Glowing Etched Bloodstone. Right click on a bloodstone to get a reward window allowing you to choose from any Rank 2 of that level.

Rank 3

Raids in Terror of Luclin drop Minor-Glowing Shadowscribed Parchment. Right click on a parchment to get a reward window allowing you to choose from any Rank 3 of that level.